Computer artists often create parameterized graphical objects for use in computer-generated material in films, television, computer games or other medium. An example is a digital 3D character (such as Shrek®), which digital 3D character is built using specialist software tools for 3D model creation. Most 3D characters are built with the intention of animation—that is, moving and deforming them in time. Manually recreating a digital 3D model for every frame of an animation would be impractical, so 3D artists build characters (or other models) with sets of controls. For the purposes of animation, the animator can set the controls to particular values and the computer modeling software will position and/or deform the 3D model according to whatever the artist intended each control to change. The controls usually have easily interpreted meanings, so for example, there may be a control to ‘raise the left eyebrow’; by changing the value of the control the animator can set the position of the eyebrow on the 3D model. Sometimes controls have more general effects such as ‘look happy’ which might change the entire character. A character can typically have anything between ten and several hundred controls.
The process of computer animation involves setting the controls to suitable values at each frame of the animation. Even though using controls is easier than rebuilding a complete model it is still impractical to set the control values for every single frame; animators usually set the control values for a much smaller number of selected ‘key frames’ and allow the animation software to apply various interpolation techniques to ‘fill in’ the values in the frames in-between.
Often, the desired animation may have something in common with the appearance or movement of an object in the real world. For example, although Shrek® or Mickey Mouse® are fantasy characters, the way they appear and the way they move when they speak or express themselves has something in common with the way real human beings look even if that commonality is extremely exaggerated or deformed and can be difficult to put into words. An extreme example is where fantasy characters' eyes can literally ‘pop out of their heads’ to express surprise—this is actually the artist's stylistic exaggeration of a real person's ‘wide eyes’ when surprised. A key part of an animator's creativity is to define the behavior of the character with respect to expression and speech.